(1) Field of the Invention
The present invention relates to an organic light emitting diode and an organic electroluminescent device which uses the organic light emitting diode and features low resistance.
(2) Description of the Related Art
Organic light emitting diodes are served as light source of the flat display due to the characteristics of low operation voltage and high luminescence efficiency. The organic light emitting diode includes multiplicity stacking organic layers disposed between two electrodes. For applied in single-side-visible displays, the organic light emitting diode has a reflective electrode and a transparent electrode. The reflective electrode is made from metal materials with low work function to be a cathode for electron injecting. The reflective electrode can improve the brightness due to reflecting light toward the transparent electrode.
However, the ambient light, which travels into the organic light emitting diode, is reflected by the reflective electrode, so as to reduce the contrast ratio of the flat display. In the light-room or under the sunlight, the contrast ratio has more influence on image quality than the luminance. Therefore, the producers hope to reduce the reflectivity of the ambient light in the display to increase the contrast ratio.
One way to improve contrast ratio is to adhere an anti-reflective film to the outer surface of the flat display. This way must add an adhering step and increase cost in the process of manufacturing the organic light emitting diode.
Another way to improve the contrast ratio is to employ an electrode with characteristics of anti-reflectance or light-absorbance, i.e. black cathode or black anode. The black cathode or anode has multiplicity films such as Al (1 nm)/metal oxide film or semiconductor film/Al (100 nm) to form the phase difference of ¼ wavelength. However, the middle layers such as metal oxide films or semiconductor films are not perfectly electric conductors. Their higher energy gap hinders flow of electrons or holes. When the panels have larger size, the higher electric resistance or the unstable electric current happens. That results in heat effect, so the temperature of the panel increases.